Conventionally, a throttle control apparatus operates a throttle valve to control an amount of air flowing into an internal combustion engine according to JP-A-2002-371866 and JP-A-2003-206762. Recently, a throttle body of the throttle apparatus is formed of resin for reducing weight to improve fuel efficiency according to JP-A-2001-303983.
According to a prior art shown in FIGS. 10 to 12, a throttle apparatus is constructed of a throttle body 101, a throttle valve, a throttle shaft 102, a power unit, a coil spring 103 and an engine control unit. The throttle body 101 internally forms an air intake passage that communicates with cylinders of the engine. The throttle valve is formed in a disc shape to open and close the air intake passage. The throttle shaft 102 is formed in a round bar shape, and the throttle shaft 102 is rigidly connected with the throttle valve. The power unit rotates the throttle valve in the opening direction thereof. The coil spring 103 urges the throttle valve in the closing direction thereof. The engine control unit operates the power unit to control opening degree of the throttle valve to be in a predetermined degree in accordance with an accelerator position operated by the driver to control the engine.
The power unit is constructed of a motor 104 and a reduction gears to rotate the throttle valve and the throttle shaft 102. The reduction gears reduce rotation speed of the driving motor 104 by a predetermined reduction ratio. The reduction gears are constructed of a pinion gear 112, a middle reduction gear 113 and a valve gear 114. The pinion gear 112 is fixed to a motor shaft 111 of the motor 104. The middle reduction gear 113 engages with the pinion gear 112 to be rotated by the pinion gear 112. The valve gear 114 engages with the middle reduction gear 113 to be rotated by the middle reduction gear 113.
A sensor cover 106 is provided to the outer wall face of a cylindrical bore wall 121 of the throttle body 101. The sensor cover 106 accommodates a throttle position sensor (rotation angle detecting device) 105 to detect an opening degree of the throttle valve (valve angle, rotation angle). The sensor cover 106 is integrally formed of thermoplastic.
The throttle control apparatus supplies electricity to the motor 104 in accordance with a signal transmitted from the accelerator position sensor. Rotation power of the motor 104 is transmitted to the throttle shaft 102 via the reduction gears, so that the throttle valve is controlled at a predetermined opening degree. Thus, intake air, which is respectively introduced into cylinders of the engine, is controlled at a predetermined amount.
As shown in FIGS. 11 and 12, a bottomed motor housing 122 is integrally formed with the outer wall of the cylindrical bore wall 121 of the throttle body 101. The motor housing 122 has a motor receiving hole 123 to receive the motor 104 that is a power source of the throttle valve. The throttle body 101 has a has a motor insertion hole 124, through which the motor 104 is inserted into the motor receiving hole 123. The outer face of the bore wall 121 has a throttle hole 125, through which one axial end of the throttle shaft 102 penetrates.
The motor 104 is inserted into the motor receiving hole 123 through the motor insertion hole 124, and a metallic plate 107 is screwed to the motor housing 122. The metallic plate 107 has a fixing hole 131, to which a cylindrical bearing holder 126 of a main frame of the motor 104 is fixed. The motor housing 122 has multiple bosses 133 that respectively have female holes 132, into which tapping screws 109 are screwed.
However, in the conventional structure shown in FIG. 12, a resinous material forming the throttle body 101 may creep after a long period, and screwing force of the tapping screws 109 with respect to the female holes 132 of the bosses 133 may decrease, after a use of the throttle apparatus for a long period. The resinous material forming the throttle body 101 may also creep after a long period. In this case, screwing force of bolts may also decrease with respect to the female holes 132 of the bosses 133, when the bolts are used for screwing the metallic plate 107 to the throttle body 101.
As a result, the position of the motor 104 becomes unstable in the motor housing 122, when screwing force of the tapping screws 109 or the bolts decreases. The pinion gear 112, which is fixed to the motor shaft 111 of the motor 104, cannot properly engage with the middle reduction gear 113. Accordingly, rotation power of the motor 104 is not smoothly transferred to the valve gear 114 that is fixed to one axial end of the throttle shaft 102, and opening degree of the throttle valve cannot be precisely controlled. Therefore, an amount of intake airflow cannot be controlled in accordance with the accelerator position, and reliability of the throttle apparatus may be degraded. Here, nuts may be insert-molded in the motor housing 122 to restrict screwing force of the bolts from decreasing. However, in this structure, the number of components is increased, and production cost is increased.